Speed limiting governors have long been used to prevent over-speed conditions in rotary pneumatic tools such as portable grinders. An unsafe overspeed condition occurs when the grinding wheel is rotated at a speed where inertia forces exceed the strength of the grinding wheel material causing the wheel to disintegrate.
The conventional governor used with a rotary pneumatic tool operates to control the speed of the tool in a no-load condition. Typically, a governor includes pivotable weights mounted about a body connected to the rotor of the machine and cooperating with a shiftable valve element in the compressed gas passageway to the rotor. The weights in the governor are selected to provide a maximum speed well below the speed at which a given grinding wheel will disintegrate.
It is desirable to increase safety even further by providing an over-speed shut-off mechanism in addition to a speed limiting governor. An over-speed shut-off mechanism operates in the event the speed governor malfunctions and causes an over-speed condition. The over-speed shut-off mechanism is designed to remain inactive as long as the governor operates correctly, but in the event the governor becomes incapable of keeping the speed below the safe limit, due to wear or seizure, the shut-off mechanism becomes activated and stops the tool.
The shut-off mechanism does not activate unless the governor has malfunctioned, so it is also preferable to require disassembly of the tool to reset the shut-off mechanism. The operator can then make necessary repairs to the governor as well as reset the shut-off mechanism.